Fix AccessWidth and BitWidth parsing in SPCR table

The ACPI Specification defines a Generic Address Structure (GAS),
which is used to describe UART controller register layout in the
SPCR table. The driver responsible for parsing it (uart_cpu_acpi)
wrongly associates the Access Size field to the uart_bas's regshft
and the register BitWidth to the regiowidth - according to
the definitions it should be opposite.

This problem remained hidden most likely because the majority of platforms
use 32-bit registers (BitWidth) which are accessed with the according
size (Dword). However on Marvell Armada 8k / Cn913x platforms,
the 32-bit registers should be accessed with Byte granulity, which
unveiled the issue.

This patch fixes above by proper values assignment and slightly improved
parsing.

Note that handling of the AccessWidth set to EFI_ACPI_6_0_UNDEFINED is
needed to work around a buggy SPCR table on EC2 x86 "bare metal" instances.

Reviewed by: manu, imp, cperciva, greg_unrelenting.technology
Obtained from: Semihalf
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D25373